Accretionary Lapilli in Volcanic Rocks of the Western Continental United States

Accretionary lapilli have been recognized at sixteen localities in California, Oregon, Nevada, Washington, Arizona, New Mexico, and Idaho. Although such lapilli from this area have heretofore been only briefly described, they are probably rather common among the silicic air-fall tuffaceous rocks of the western United States. The accretionary lapilli are pea-size structures composed entirely of clastic volcanic material, primarily glass or its alteration products. The lapilli exhibit a distinct decrease in grain size from core to rim. All the accretionary lapilli studied apparently formed in the air during volcanic eruptions. They probably developed in an ash-charged volcanic cloud by accretion of ash around a core due to the condensation of moisture on the core, and fell to the ground like hailstones. Accretionary lapilli provide information for stratigraphic studies, on the nature of the volcanism that produced them, and on post-depositional changes that have affected the rock which contains them.